This is a nut housing for round brass nuts used with trapezoidal stainless steel rods (in my case 12x3mm). The nut's diameter is 26mm and its height 24mm (see http://www.maedler.de/article/64331200).
I printed 2 sets of these for my small DIN A4 ("2030") DIY CNC mill/laser machine to properly fix the nuts of the acme rods to the Y table and X axis. The semi-open design can be used to slide the front half of the enclosure between mounted table and nut. The "back part" is the same.
2 M6 bolts and nuts are needed to assemble and at the same time attach the part to your axis.
I left about half a millimeter of air between the acrylic plate and the enclosure to slide a thin aluminum sheet in between to fine-tune the axis. For the Y table 3 sheet layers were needed.
For more info on the machine visit
I'm currently using it with the newest version of GRBL for machining plywood. The CAD/CAM chain is Google Sketchup 8 with SketchUCam V1.1e-beta plugin, which makes nice, grbl-friendly Gcode.